For electrical and electronic component testing, a test fixture is a convenient and reliable way of connecting a “device under test” (“DUT”) to test equipment. The test fixture is designed to remove measurement current from the sensing circuit, thereby eliminating spurious voltages from the sensing circuit. Typically, a four-terminal-pair (“4TP”) test fixture is used to measure impedance of electronic components. The 4TP test fixture attempts to measure the voltage across the device under test and the current flowing therethrough. One pair of terminals provides an excitation signal. A second pair of terminals measures voltage. A third pair of terminals is used to provide feedback for maintaining a virtual ground at one terminal of the DUT; and a fourth pair of terminals is used to source the current needed to effect the virtual ground and to measured the sourced current (which is equal to the current through the DUT). The process of maintaining a virtual ground at one end of the DUT is referred to as the auto-balancing bridge (ABB) technique.
The impedance of the DUT is calculated from the relationship Z=V/I, where Z is the impedance of the DUT, V is the voltage measured across the DUT, and I is the current flowing through the DUT. The measurement current is normally sinusoidal, and includes both magnitude and phase components for current and voltage. From these measurements, both the resistive and reactive components of the DUT are determined. For example, the impedance of an inductor consists of both its inductive and resistive components, j, ω, L and R.
The test fixture typically incorporates stray impedance. In cases where the stray impedance is significantly less that the impedance of the DUT, it is often ignored. The stray impedance can be reduced by calibration procedures. The simplest calibration procedure involves temporarily replacing the DUT with a short, and then ‘zeroing’ test equipment. For compensation of parallel stray impedances, the test fixture can be used with no DUT or short in place—an “open” calibration. A known impedance, usually resistive, can also be used for a third calibration step, referred to as “load” calibration.
A wide range of commercially available test fixtures are used throughout industry; they are often designed for ease of use, and accept a wide range of DUTs.
Recent improvements in test equipment performance allow low impedances to be measured more accurately. However, the stray impedance of most test fixtures significantly limits the accuracy of these measurements. The way in which the DUT is mounted contributes significantly to the stray impedance. Most commercially available 4TP test fixtures have stray impedance levels that prevent accurate measurement of low impedances, even after calibration.